The present invention relates to communications methods and apparatus, and more particularly, to spread spectrum communications methods arid apparatus.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication for a cell 24. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple-channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i. e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use xe2x80x9cdirect sequencexe2x80x9d spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
In a typical CDMA system, a data stream intended for a particular user (terminal) is first direct-sequence spread according to a user-specific spreading sequence, and the resultant signal is then scrambled according to a cell-specific scrambling sequence. The spread and scrambled user data stream is then transmitted in a communications medium. Spread-spectrum signals for multiple users generally combined to form composite signal in the communications medium.
Downlink signals for different physical channels within a cell are typically transmitted from a base station in a synchronous fashion. The user-specific spreading codes are typically orthogonal, creating mutually orthogonal downlink signals at the transmitter. However, channel dispersion typically results in a loss of orthogonality at the receiver, giving rise to intra-cell multi-user interference that can lead to degradation of receiver performance. This interference can be exacerbated by the xe2x80x9cnear-farxe2x80x9d problem, i.e., the higher contribution of signal energy from interferers that are closer to the receiver than the station transmitting a desired signal. Although the near-far problem can be alleviated by power control techniques on the uplink, power control is generally impractical on the downlink.
These problems may be exacerbated in xe2x80x9cthird generationxe2x80x9d systems such as wideband CDMA (WCDMA) systems. Such systems typically are intended to support several kinds of communications services, including voice and data applications that have varying information rate requirements. Generally, these systems are designed to support higher data rates than their predecessors, and also are designed to support a wide variety of data rates. However, the higher data rates and increased bandwidth of such wideband systems can combine to cause severe inter-cell and intra-cell interference among users. Such wideband third-generation systems are typically designed to support multiple spreading factors, which means that signals with low spreading factors generally require higher transmit power to achieve the same link quality as signals transmitted using higher spreading factors. This power differential can further increase intra-cell multiuser interference.
Several techniques for canceling such interference have been proposed, but many of these are more suitable for uplink interference cancellation, i.e., are highly complex and/or presuppose a priori knowledge of the spreading sequences being used in the system. Although such techniques can provide effective interference cancellation, in the presence of increasing demand for wireless services there is an ongoing need for interference cancellation techniques to further improve resource utilization.
In light of the foregoing, it is an object of the present invention to provide methods and apparatus for recovering information in composite spread spectrum signals transmitted in a communications medium.
It is another object of the present invention to provide methods and apparatus for recovering information transmitted according to a spreading code in the presence of interference from other spread spectrum signals.
It is a further object of the present invention to provide methods and apparatus for recovering a selected user signal in a cellular CDMA system in the presence of intra-cell multiuser interference.
These and other objects, features and advantages may be provided, according to the present invention, by methods and apparatus in which a baseband signal is recursively modified based on an estimate of interfering signal components constructed from spreading sequences detected in the baseband signal. More particularly, in embodiments according to the present invention, the baseband signal is descrambled to into multipath components that are correlated with a set of spreading sequences, for example, a set of possible spreading sequences used in a wireless CDMA communications system, yielding sets of correlations that are maximal ratio combined (MRC) to detect spreading sequences associated with interfering signal components in the baseband signal. Associated symbol estimates may be generated from the MRC results, and processed according to the detected spreading code, scrambling code and a channel estimate to generate an estimate of the interfering signal component. The estimate of the interfering signal component may then be used to cancel interference in the baseband signal using, for example, subtraction or signal projection techniques.
In one embodiment according to the present invention, desired information is recovered from a communications signal that includes interference by processing the communications signal to produce a baseband signal and detecting a spreading sequence from the baseband signal. An estimate of an interfering signal component of the baseband signal associated with the detected spreading sequence is detected. A new version of the baseband signal is generated from a previous version of the baseband signal based on the generated estimate of the interfering signal component. The desired information is estimated from the new version of the baseband signal.
The spreading sequence preferably is detected by resolving, e.g., descrambling, a version of the baseband signal into a plurality of signal components, respective ones of which are associated with respective propagation paths. The resolved plurality of signal components are correlated with a set of spreading sequences to generate respective sets of correlations for respective ones of the resolved signal components. The correlations are combined, preferably using MRC, to generate a plurality of combined correlations, respective ones of which correspond to respective ones of the spreading sequences. The spreading sequence associated with the interfering signal component is detected from the generated combined correlations. For example, the detected spreading sequence may be the sequence having the highest energy as indicated by the combined correlations, indicating that the interfering signal component associated with the detected signal has the highest signal strength in comparison with other interferers in the baseband signal.
The estimate of the interfering signal component preferably is generated by generating an estimate of a symbol transmitted according to the detected spreading sequence from the combined correlation associated with the detected spreading sequence. The estimate of the symbol is then processed according to the detected spreading sequence, the scrambling sequence and a channel estimate to generate an estimate of the interfering signal component.
According to another embodiment of the present invention, a plurality of spreading sequences are detected from the resolved plurality of signal components, respective ones of the detected plurality of spreading sequences being associated with respective ones of a plurality of interfering signal components in the communications signal. An estimate of the interfering signal component is then generated by generating respective estimates of symbols encoded according to respective ones of the detected spreading sequences from the combined correlations associated with the detected spreading sequences. Respective ones of the estimates of the encoded symbols are spread according to respective corresponding spreading sequences, and the spread estimates of the encoded symbols are combined to generate a composite signal. The composite signal is scrambled according to the scrambling sequence and processed according to a channel estimate to generate a composite estimate of the plurality of interfering signal components. A new version of the baseband signal is generated from a previous version of the baseband signal and the generated composite estimate of the plurality of interfering signal components. The new version of the baseband signal may be generated, for example, by subtracting the estimate of the interfering signal component from the previous version of the baseband signal to generate the new version of the baseband signal, or by determining a projection of the previous version of the baseband signal in a direction in signal space orthogonal to the estimate of the interfering signal component.
According to a xe2x80x9csubspace cancellationxe2x80x9d aspect of the present invention, respective ones of the estimates of the encoded symbols are spread according to respective corresponding spreading sequences. The spread estimates of the encoded symbols are then scrambled separately according to the scrambling sequence to generate a plurality of scrambled spread signals. The scrambled spread signals are then separately processed according to the channel estimate to generate respective estimates of respective ones of the plurality of interfering signal components. A new version of the baseband signal is generated from a previous version of the baseband signal and the generated estimates of the plurality of interfering signal components, preferably by determining a projection of the previous version of the baseband signal in a direction in signal space orthogonal to the generated estimates of the plurality of interfering signal components. The projection may be determined using, for example, a Gram-Schmidt orthogonalization.
According to another aspect of the present invention, the recursively generated baseband signal may be used to estimate desired information in the baseband signal using similar techniques to those used to generate the estimates of interfering signal components. The new version of the baseband signal is resolved into a plurality of interference-canceled signal components, and the resolved plurality of interference-canceled signal components correlated with a desired spreading sequence to generate a set of correlations for the resolved signal components. The correlations are then combined and the desired information estimated from the combined correlations.
According to yet another embodiment of the present invention, spreading sequences are detected for selected symbol periods, and used to generate estimates of interfering signal components for other symbol periods. A spreading sequence is detected from a first portion of the baseband signal corresponding to a first transmitted symbol. An estimate of an interfering signal component is generated from a second portion of the baseband signal associated with a second transmitted symbol based on the detected spreading sequence. A new version of the second portion of the baseband signal is estimated from a previous version of the second portion of the baseband signal and the generated estimate of the interfering signal component, and the second transmitted symbol is estimated from the new version of the second portion of the baseband signal.
According to yet another aspect of the present invention, an apparatus for recovering information from a communications signal comprises means for processing the communications signal to produce a baseband signal. Means for detecting a spreading sequence from the baseband signal are provided, along with means for generating an estimate of an interfering signal component of the baseband signal associated with the detected spreading sequence. Means are also provided, responsive to the means for generating an estimate of an interfering signal component, for generating a new version of the baseband signal from a previous version of the baseband signal based on the generated estimate of the interfering signal component. In addition, means are provided, responsive to the means for generating a new version of the baseband signal, for estimating the desired information from the new version of the baseband signal.
According to another aspect of the present invention, a receiver includes a recursive baseband processor that detects a spreading sequence from the baseband signal, generates an estimate of an interfering signal component of the baseband signal associated with the detected spreading sequence, generates an interference-canceled version of the baseband signal from the baseband signal based on correlations of the baseband signal with a set of spreading sequences, and generates an estimate the desired information from the interference-canceled version of the baseband signal. The baseband processor preferably includes a descrambler operative to resolve a version of the baseband signal into a plurality of multipath signal components. A correlator is responsive to the descrambler, correlating the resolved plurality of multipath signal components with a set of spreading sequences to generate respective sets of correlations for respective ones of the resolved signal components. A maximal ratio combiner is responsive to the correlator and operative to combine correlations to generate a plurality of combined correlations, respective ones of which correspond to respective ones of the spreading sequences.
A spreading sequence detector is responsive to the maximal ratio combiner and operative to detect the spreading sequence associated with the interfering signal component from the generated combined correlation. A symbol estimator is also responsive to the maximal ratio combiner and operative to generate a symbol estimate for the interfering signal component. A spreader is responsive to the spreading sequence detector and to the symbol estimator, and spreads the generated symbol estimate according to the detected spreading sequence. A scrambler is responsive to the spreader and operative to scramble the spread symbol estimate according to the scrambling sequence. A channel emulator is responsive to the scrambler and operative to process the scrambled spread symbol estimate according to a channel estimate to generate an estimate of the interfering signal component. A baseband interference canceler is responsive to the channel emulator, generating a new version of the baseband signal from a previous version of the baseband signal and the generated estimate of the interfering signal component.
In another embodiment according to the present invention, the descrambler is further operative to resolve the new version of the baseband signal into a plurality of signal components, respective ones of which are associated with respective propagation paths. The correlator is further operative to correlate the resolved plurality of signal components of the new version of the baseband signal with a desired spreading sequence to generate a set of correlations for the resolved signal components of the new version of the baseband signal. The maximal ratio combiner is further operative to combine the set of correlations for the resolved signal components of the new version of the baseband signal to generate a decision statistic for a symbol transmitted according to the desired spreading sequence. The receiver further comprises a symbol estimator responsive to the maximal ratio combiner and operative to generate an estimate of a symbol transmitted according to the desired spreading sequence from the generated decision statistic.